High-speed data transmission is the foundation of modern information society, the requirement of the data capacity that transmits in a fiber gets bigger and bigger with the massive growth of the amount of information. In addition to increasing modulation rate and using more wavelengths, bidirectional transmission in one optical fiber, which use the bidirectional optical transceiver module with low cost, can double the data transmission capacity in one fiber, this is an effective and adopted method in the optical communication industry.
In addition, the accuracy requirement of clock synchronization for modern communication network increase highly. The traditional optical transceiver module transmits and receives the optical signals by using two optical fibers, respectively, the length of the two optical fibers in the practical application will cause the propagation delay of the two signals inconformity, which causes huge difficulty to the clock synchronization. The use of single optical fiber bidirectional transmission can meet the accuracy requirement of the clock synchronization.
The generally adopted single-fiber bidirectional optical transceiver module scheme is as shown in FIG. 1. The transmission and receiving modules of the optical transceiver modules (102) and (103) have different configuration, such as the transmitting wavelength of the optical transceiver module (102) is λ1, and the receiving wavelength is λ2; the transmitting wavelength of the optical transceiver module (103) is λ2, and the receiving wavelength is 1. The wavelength filters (104) and (105) in the optical transceiver modules (102) and (103) have different optical filter properties, such as the wavelength filter (104) transmits λ1, and reflects λ2; wavelength filter (105) transmits λ2, and reflects λ1.
It can be seen that the single-fiber bidirectional transmission with dual-wavelength scheme in the existing technology shown in FIG. 1 needs to prepare two different types of optical transceiver module to be paired in the practical application. This is not only increase the inventory pressure, but also increase a certain difficulty in field deployment. Besides, due to the effect of the dispersion, the two different wavelengths also have a certain delay inequality even being transmitted in the same optical fiber, which can not meet the demand for high accuracy of clock synchronization in the application scenario.
Using the same wavelength in the bidirectional transmission of the same optical fiber is the way to solve the problems. In the existing technical solution as shown in FIG. 2 (Chinese Patent Application No. 201110282629.6), the transmitting and receiving of the optical transceiver modules (202) and (203) of the optical fiber ends adopt the same wavelength λ, and use the beam splitters (204) and (205) to replace the wavelength filter in scheme 1. The function of the beam splitter is partially reflecting and partially transmitting the optical signal that incident on it, the splitting ratio is usually 50% to 50%. The redundant reflected light generated by the beam splitter (204) and (205) is absorbed by the black light absorption body (206) and (207), which can avoid the crosstalk to the system. Thus, the optical transceiver module (202) and (203) of the optical fiber ends are the same, and no need to be paired.
The existing technology shown in FIG. 2 has a significant deficiency, that is the beam splitter (204) and (205) can produce a total of 6 dB link loss. And in many applications, the extra 6 dB loss is unacceptable.
To avoid the extra loss problem of the technical solution shown in FIG. 2, Chinese Patent Application 201110373606.6 disclosures a technical solution, as shown in FIG. 3. The technical solution adopts a set of optical elements (303) to (310), which enable the transmitting and receiving of the same wavelength. Specifically, the optical signal inputted to the input/output end (302) of the optical transceiver module (300) transmitted from the optical fiber (301) usually contains the first and the second polarization-states (using “|” and “⋅”), after passing through the first polarization beam splitter (303), two polarization-states separates; the optical signal of the first polarization-state arrives at the optical receiver (312) after passing through the Faraday rotator (305), half wave plate (307), the second polarization beam splitter (308), half wave plate (309) and the third polarization beam splitter (310); and the optical signal of the second polarization-state arrives at the optical receiver (312) after passing through the mirror (304) and the third polarization beam splitter (310).
The optical signal emitted from the optical transmitter (311) is with the first polarization-state (“|”), which arrives at the input/output end (302) after passing though the second polarization beam splitter (308), half wave plate (307), Faraday rotator (305) and the first polarization beam splitter (303).
Magnetic ring provides the magnetic field required by the Faraday rotator (305).
The technical solution shown in FIG. 3 enable the single-fiber bidirectional transmission of the same wavelength, which avoids the excessive power loss. However, the technical solution adopts overmuch optical element, which increases the cost; and two polarization-state transmission paths have larger separation in space, which also causes the volume difficult to further reducing. U.S. Pat. No. 7,039,278B1) also discourses the similar structure in scheme as shown in FIG. 3, which has the same problems of the volume and cost.
U.S. Pat. No. 7,039,278B1) further discourses a relatively compact structure, as shown in FIG. 4. The inputted optical signal of the transceiver module's (400) input/output end (401) incident on the first polarization beam splitter (403) after collimating by the first collimating lens (402), and disintegrating into the first polarization-state optical signal and the second polarization-state optical signal that are perpendicular to each other, and the second polarization-state optical signal is reflected by the second polarization beam splitter (403), and then reflected by the group of a ¼ wave plate (404) and a mirror (405), and the polarization-state rotates 90-degree, transmitting through the first polarization beam splitter (403), and passing through the second collimating lens (406), converging and getting to the optical detector (407), and receiving it.
The disintegrated first polarization-state optical signal by the first polarization beam splitter (403) transmitting through the first polarization beam splitter (403) rotates 90-degree after passing through the ½ wave plate (408) and the Faraday rotator (409), and reflected to the mirror (411) on the second polarization beam splitter (410), which reflected by the mirror (411) and the second polarization beam splitter (410) again, passing the Faraday rotator (409) and the ½ wave plate (408) in reverse direction, for the non-reciprocity of the Faraday rotator (409), the returned optical signal is perpendicular to the polarization-state of the first polarization-state optical signal, thus reflected by the first polarization beam splitter (403), and through the second collimating lens (406), converging and getting to the optical detector (407), and receiving it.
The outputted optical signal emitted form the laser chip (413) has the single polarization-state, after becoming the collimated beam through the third collimating lens (412), transmitting through the second polarization beam splitter (410), and through the Faraday rotator (409) and ½ wave plate (408), the polarization-state keeps the same, and further transmitting through the second polarization beam splitter (403), outputting to the input/output end that focuses through the first collimating lens (402).
The above scheme still uses a plurality of optical elements, causing big volume and high cost, and assembly difficulty. Furthermore, compared with the second polarization-state optical signal, the first polarization-state optical signal propagate back and forth through the second polarization beam splitter twice more, and the time of getting to the optical detector (407) has difference, causing a great polarization mode dispersion. The polarization mode dispersion depends on the size and refractive index of the second polarization beam splitter, if the refractive index is 1.5, even with the smallest size of 1 mm, the generated polarization mode dispersion will be about 10 ps, which is not suitable for high speed signal receiving (more than 10 G).
US Patent Application US20140054657, US20080042050 and the above U.S. Pat. No. 7,039,278B1 also revealed using a pair of birefringent crystals, inserting half wave plate and a Faraday rotator in the middle to separate the transmitting and receiving channels, however, the basic limitation is the distance of the transmitting and receiving channels is in direct proportion to the length of the birefringent crystals, to make the position of the transmitting and receiving channels having enough separation, the length of the optical element is close to 10 mm.
U.S. Pat. No. 7,039,278B1 also revealed using a pair of birefringent crystals wedge, adding the Faraday rotator in the middle, which can separate the transmitting and receiving channels in the form of angle separation, and use an optical lens to transform the angle separation into the location separation for the transmitting laser and optical detector. However, the scheme needs integrated transmitting and receiving chip, the process isn't easy to realize and the cost is high. Besides, the transmitting and receiving chip are too close, the reflected light from the lens surface, which is originally emitted from the transmitting laser chip, can easily get into the optical detector; for the power of optical signal emitted from the transmitting laser chip is very high, so even a small reflection will produce unacceptable crosstalk to the receiving end.
In conclusion, the existing single-fiber bidirectional transmission solution with the same wavelength has the lack of performance, size, and cost more or less, therefore, a new technology of low cost without larger crosstalk and loss, and small sized single-fiber bidirectional optical transceiver module with the same wavelength is needed. Particularly, if the geometric structure and size provided by the new technology is compatible with the existing technology shown in FIG. 1, which can maximally use the existing platform, significantly lower the cost, and share the simple and convenient of communication system bring by the technology of single-fiber bidirectional with dual-wavelength.